1. Field of the Invention
The present invention relates to a medical-device magnetic guiding position detecting system.
2. Description of Related Art
In recent years, a medical device that is swallowable by a subject, such as an examinee, has been researched and developed for practical use. The medical device is capable of passing through a body lumen of the subject so as to capture an image of a target location in the body lumen. An example of the medical device is a swallowable endoscopic capsule.
In order to guide such a medical device to the target location in the body lumen, means for detecting the current position of the medical device in the body lumen and guiding the medical device to the target location is required.
As the means for guiding the medical device to the target location, means is known in which a magnet is mounted in an endoscope and the position of the endoscope is controlled by externally applying a magnetic field to the magnet.
To detect the position of the medical device, a method for magnetically detecting the position is known. A magnetic position detecting method is known in which a magnetic field is externally applied to a subject having a coil therein and a magnetic field generated by an electromotive force induced by the applied magnetic field is detected using an external magnetic sensor. Refer to, for example, the following Document 1 and Document 2:    Document 1: Japanese Unexamined Patent Application, Publication No. Hei 6-285044    Document 2: “Precision Position-Detecting System Using Wireless LC Resonant Magnetic Marker”, TOKUNAGA, HASHI, YABUKAMI, KONO, TOYODA, OZAWA, OKAZAKI, and ARAI, Journal of the Magnetic Society of Japan, vol. 29, p 153-156, 2005.
Document 1 describes a technology in which a substantially rectangular parallelepiped magnetic field source (a magnetic field generating coil for detecting a position) including three magnetic field generating coils whose axes are perpendicular to each other is disposed outside a body and, a magnetic field detecting coil including three magnetic field reception coils whose axes are perpendicular to each other is disposed in a medical capsule. According to this technology, the magnetic field source generates an alternating magnetic field, which induces an electrical current in the magnetic field detection coil. The position of the magnetic field detecting coil, i.e., the position of the medical capsule can be detected on the basis of the induced electrical current.
In contrast, Document 2 describes a position detecting system including an exciting coil (a position detecting magnetic field generating coil) that generates an alternating magnetic field, an LC resonant magnetic marker that generates an induced magnetic field in response to the reception of the alternating magnetic field, and a detection coil that detects the induced magnetic field. In this position detecting system, the LC resonant magnetic marker resonates at a predetermined frequency in accordance with an additional capacitance and a parasitic capacitance. Accordingly, if the frequency of the alternating magnetic field is set so as to be equal to the predetermined frequency, the strength of the induced magnetic field can be significantly increased compared with another frequency, and therefore, the detection efficiency can be increased.
However, in the case where the technologies described in Documents 1 and 2 are combined with an technology in which a magnetic field is used for guiding the medical capsule, if the center axis line of a guiding magnetic field generating coil for generating a guiding magnetic field is substantially coincident with that of the position detecting magnetic field generating coil, mutual induction may occur between the position detecting magnetic field generating coil and the guiding magnetic field generating coil in accordance with a time-varying change in the alternating magnetic field generated by the position detecting magnetic field generating coil.
That is, an electromotive force generated in the guiding magnetic field generating coil by the mutual induction causes an electrical current to flow in a closed circuit formed by the guiding magnetic field generating coil and a driving unit for driving the guiding coil. This electrical current disadvantageously generates a magnetic field that cancels out the above-described alternating magnetic field.
In addition, in order to make the distribution of a magnetic field in a guiding space uniform, the structure of the guiding magnetic field generating coil, in general, is of the Helmholtz type or a type similar to the Helmholtz type. In general, two guiding magnetic field generating coils are connected to the driving unit of a guiding coil in series. In such a case, even when an electromotive force due to the mutual induction is generated in only one of the two guiding magnetic field generating coils, a closed circuit is formed by the driving unit of a guiding coil. Accordingly, an electrical current flows in the other guiding magnetic field generating coil. For this reason, a magnetic field having a phase substantially opposite to that of the position detecting magnetic field is widely distributed in the guiding space.
At that time, as shown in FIG. 21, a combined magnetic field (shown by a solid line C) composed of the position detecting magnetic field (shown by a dotted line A) emanating from the position detecting magnetic field generating coil and the guiding magnetic field (shown by a dotted line B) emanating from the guiding magnetic field generating coil passes through the coil incorporated in the capsule. In particular, depending on a relationship between the positions of the position detecting magnetic field generating coil and the guiding magnetic field generating coil, the position detecting magnetic field (shown by the dotted line A) could be substantially completely canceled out by the guiding magnetic field (shown by the dotted line B) in an area (L) even within the operational area of the medical device. Since no magnetic fields pass through the coil incorporated in the capsule, no induced electrical currents flow in the coil. Therefore, no induced magnetic fields occur in the coil. As a result, the position of the medical capsule cannot be detected in that area.